A titanium dioxide has a high-refractive index and excellent whiteness, hiding power, and tinting power, and has therefore been widely used as a white pigment for paints, plastics, and the like. Further, the titanium dioxide can also be used for, for example, cosmetics and catalysts as a substance for shielding ultraviolet rays such as an ultraviolet ray absorbing agent or an ultraviolet ray shielding agent by controlling a particle size or photoactivity of the titanium dioxide. For that reason, in recent years, research and development has been actively conducted on such use of the titanium dioxide.
It has been known that when a titanium dioxide powder having an apparent specific average particle size, which comprises small spherical particles of a titanium dioxide having a specific average primary particle size and a spherical moss-like shape made up of a large number of titanium dioxide, is used for cosmetics, the titanium dioxide powder serves as a functional material capable of imparting a good slippery property and excellent light fastness that conventional titanium dioxides do not have (Patent Literature 1).
Further, a cosmetic for lips containing 1 to 15% by mass of rutile titanium oxide aggregated particles having an average particle size of 0.2 to 0.4 μm and an average coefficient of friction (MIU value) of 0.4 to 0.6 and 1 to 40% by mass of semisolid oil has been known to give luster, cover noticeable wrinkles of lips, and provide excellent long-lasting makeup properties (Patent Literature 2).
Moreover, it has been known that a cosmetic containing a coloring material having a small absorbance of light on a long wavelength side (wavelength of 630 to 700 nm) in a visible light region makes the skin look close to the bare skin due to a light transmitting property inside of the skin, and thus a natural makeup finish can be achieved (Patent Literature 3).
While the titanium dioxide has a high-refractive index and high-hiding power to hide pigmented spots and the like of the skin, blending a large amount of titanium dioxides to increase the hiding power results in providing an unnatural makeup finish (effect) and thus irregularities on the skin may become more noticeable as compared to those on the bare skin before application. This is because as the bare skin has a light transmitting property and has a large amount of light scattered and/or fed back from inside the skin, as described above, shadows are unlikely to be created on tiny irregularities on the skin and that makes the skin look natural. However, as the titanium dioxide reflects a much larger amount of light by its surface, light is prevented from transmitting inside the skin and thus shadows are created on the irregularities on the skin, which makes the irregularities on the skin noticeable. For that reason, it is desired to develop a titanium oxide designed to further improve the transmitting property of light on the long wavelength side while having the hiding power equivalent to a commonly-used titanium oxide pigment.
As described above, as the titanium oxide having an improved light transmittance for light on the long wavelength side, a rutile titanium oxide in the shape of strips or straw bundles has been developed which has a particulate form formed by bundle-like orientation and aggregation of rod-shaped particles, where the formed particle has an apparent average length of 80 to 300 nm, an apparent average width of 30 to 150 nm, an apparent average axial ratio represented by the apparent average length/the apparent average width of 1.1 to 4, and a specific surface area of 120 to 180 m2/g. This titanium oxide has been known to have a high-level of transparency and ultraviolet ray shielding performance (Patent Literature 4).
However, because this titanium dioxide is an aggregate of rod-shaped particles and has many voids in the secondary aggregate, the titanium dioxide has a decreased apparent refractive index and thus an insufficient hiding power when it is actually added to cosmetics. And because the titanium dioxide is intended mainly for ultraviolet protection, the secondary aggregate has an apparent particle size of less than 100 nm, which is clearly smaller than the particle size capable of maximizing the scattering effect of the titanium oxide based on Mie theory, and this is also a factor making the hiding power low.